A frequency-change detection type sensor utilizing the SAW oscillator has been known, for example, in Japanese Patent Application Publications No. 61-234324 (hereinafter, document 1) and No. 9-80035 (hereinafter, document 2). The frequency-change detection type sensor detects a change in oscillating frequency, which is determined by the SAW element and an oscillating circuit forming an oscillation loop.
In the frequency-change detection type sensor, because a resonance frequency of a SAW element is high, such as hundreds of megahertz, an expensive AD converter needs to be used to process high-frequency signals. Otherwise, the oscillating frequency is usually down-converted, for example, to tens of kilohertz, so that the signals can be processed by an inexpensive AD converter. The down-converting is carried out by obtaining a difference of oscillating frequencies of oscillators that employ SAW elements having different resonance frequencies. The difference of the oscillating frequencies is hereinafter referred to as the difference frequency.
In general, the SAW element has interdigital electrodes and reflectors formed on the surface of a piezoelectric material. To form the interdigital electrodes and the reflectors, for example, a thin metal film, such as a thin aluminum film, is formed on the piezoelectric material by sputtering or the like, and then the thin metal film is formed into desired electrode pattern by a photo-etching technique, such as by photo-lithography and etching. Further, because there is a constant relation between the interval (electrode pitch) of the interdigital electrodes and electrodes of the reflectors and a resonance frequency, the electrode pitch is usually adjusted so that the SAW elements have desired resonance frequencies.
For example, Japanese Patent Application Publication No. 6-224678 (hereinafter, document 3) describes a method of adjusting the resonance frequency of the SAW element to a desired resonance frequency. Specifically, after an electrode pattern is formed, a substrate is dry-etched using the electrode as a mask to increase the thickness of the electrode while monitoring the resonance frequency.
Japanese Patent Application Publication No. 2005-65042 describes another method of adjusting the resonance frequency of the SAW element. Specifically, after the electrode pattern is formed, side surfaces of the interdigital electrode are anode-oxidized to increase a mass of the interdigital electrode and to expand a line width of the interdigital electrode.
In order to improve a resolution capability of the frequency-change detection type sensor, jitter needs to be reduced. Therefore, it is necessary to reduce the difference frequency between the two oscillators to a minute level smaller than 500 kilohertz, such as to tens of kilohertz.
In a case where the two oscillators are constructed using the SAW elements having different resonance frequencies, it is necessary to design resonance frequencies of the SAW elements at a minute unit in order to generate such a minute difference frequency. However, it is actually difficult to control the electrode pitch less than 0.1 micrometers by a present photo-lithography technique. For example, with regard to a SAW element having the resonance frequency of 200 megahertz and using a LiNbO3 substrate, the resonance frequency can only be designed by a unit of approximately 1 megahertz. Therefore, it is difficult to design a structure of the SAW element so as to achieve the minute different frequency.
In the documents 3 and 4, the resonance frequency of the SAW element is adjusted after the SAW element is formed once. That is, the resonance frequency is not adjusted in a stage of designing the SAW element. If the adjustment of the resonance frequency is carried out after the SAW element is formed as described in the documents 3 and 4, an adjustment process becomes complex.
Accordingly, there has been no method for designing the SAW elements so that the minute difference frequency can be generated between the two oscillators.
In addition, in the conventional frequency-change detection type sensor utilizing the SAW elements having different resonance frequencies, a detection error may occur.
FIG. 14 shows a temperature property of two SAW elements having different resonance frequencies. In general, the SAW element has the temperature property relative to the resonance frequency as defined by an expression below. Therefore, the resonance frequency changes with the temperature (T).f=f0(1+αT)
Further, the temperature characteristic depends on the resonance frequency of the SAW element. Therefore, the two SAW elements having the different resonance frequencies have different temperature properties. Having the different temperature properties means that the amount of change in resonance frequency relative to a change in temperature is different. As such, as shown in FIG. 14, the difference between the resonance frequencies of the two SAW elements (Δf=f2−f1) varies depending on the temperature (Δf′=(f2−f1) (1+αT)). Accordingly, as the temperature increases, the difference between the resonance frequencies increases.
As a result, even if an input signal has no change in frequency, the resonance frequencies of the two SAW elements are changed by the change in the temperature, and thus the detection error occurs.